Interpretive Summary: Xylans are the major hemicellulose in plants, representing an abundant carbon source on earth, second only to cellulose. Many bacteria, fungi, and yeasts produce enzymes that degrade cell wall hemicellulose. The complete breakdown of xylan requires the cooperative action of several enzyme components, with the major depolymerizing enzyme being endo-xylanases. Endo-xylanases are grouped into families with different actions in degrading the xylan polymer. This report describes the discovery of a novel endo-xylanase produced by the fungus Trichoderma reesei, with a unique mechanism of cleavage of the monomeric xylose from the reducing end of the xylan polymer. This enzyme may contribute as an important component of the enzyme system to the complete degradation of hemicellulose.

Technical Abstract:
A novel xylanase from Trichoderma reesei Rut C30, named XYN IV, was purified from the cellulolytic system of the fungus. The enzyme was discovered on its ability to attack aldotetraohexenuronic acid (HexA-2Xyl-4Xyl-4Xyl, HexA3Xyl3), releasing the reducing-end xylose residue. XYN IV exhibited catalytic properties different from previously described endo-'-1,4-xylanases of this fungus, XYNI, XYNII, XYNIII, and the xylan-hydrolyzing EGI. XYN IV was able to degrade various '-1,4-xylans, but was inactive on ß-1,4-mannans and ß-1,4-glucans. It showed both exo-and endo-xylanase activity. Rhodymenan, a linear soluble ß-1,3-ß-1,4-xylan, served as the best substrate. Linear xylooligosaccharides were attacked exclusively at the first glycosidic linkage from the reducing end. The gene xyn4 encoding XYN IV was also isolated. It showed clear homology with xylanases classified in glycoside hydrolase family 30 harbouring also glucanases and mannanases. The xyn4 gene was expressed slightly on xylose and xylitol, clearly on arabinose, arabitol, sophorose, xylobiose, xylan and cellulose, but not on glucose or sorbitol, resembling induction of other xylanolytic enzymes from T. reesei. A recombinant enzyme prepared in Pichia pastoris expression system exhibited identical catalytic properties as the enzyme isolated from the T. reesei culture medium. The physiological role of this unique enzyme remains unknown, but it could be liberation of a xylose residue from the reducing end of branched oligosaccharides that are resistant toward ß-xylosidase and other types of endoxylanases. By its catalytic properties XYN IV differs from bacterial GH30 glucuronoxylanases that recognize MeGlcA substituents as substrate specificity determinants.